The reserved page has partitioning configuration information.
This call returns a pointer (physical addr) to the reserved page.

As some background, a large SGI UV system can be run as multiple
smaller systems (each smaller system is refered to as a partition).
For example, a two rack system can be run as two one rack partitions.
Each partition has a unique linux kernel (just like a stand alone system).
The reserve page is a way of sharing configuration information about
the other partitions in a partitioned system. The UV hardware allows
memory to memory transfers between partitions using the GRU (Global
Reference Unit). SGI Altix hardware has similar functionality (though
the hardware specifics are different).

This is used by the xp driver (drivers/misc/sgi-xp/xpc_uv.c).
Dean Nelson submitted a patch yesterday that uses this bios call.